I have noticed some low level ‘electrical’ type noise on my speakers of late and set about trying to track it down.
So going around the area of the amps and pre/eq etc (I run active speakers) I tracked it down to not being the amp or pre/eq (at least not the worst of it) but down to what was being used to power the RPi with 7" display and Ropieee. I had run out of power supplies for my RPi and was pulling power from a cable set top box USB port temporarily - so this was the culprit for this weird noise I was getting. Thinking I might even try to get my 5V 25VA LMPS running to power this as its connecting to 2-3 DAC’s (my Emotiva XMC-1 USB and OPPO HA-1 and Sonica DAC) tho I might replace the primary Sonica DAC connection with a microRendu at some point too.
So power supplies can wreak havoc in Hifi when you least expect. I’ve heard of light and fan dimmers/controllers also making off noise in the audio chain and also wifi/routers, cordless phones, computers, fridges, AC, microwaves and even plasma TV’s so if you hear something odd coming out of the speakers go about the house turning things off until you get some improvements then at least you have a device to blame as the cause…then you need to think about resolution.